1. Field of the Invention
The present invention relates to an internal combustion engine provided with two intake valves for each cylinder.
2. Description of the Related Art
Known in the prior art is an internal combustion engine having a first intake port of a helical shape, a second intake port of a straight shape, and a fuel injector mounted in the second intake port (for example, Japanese Unexamined Patent Publication (Kokai) Nos. 58-204959, 58-206815, 52-34108, 54-84128, 58-1725514, and 58-172424). This type of engine aims to provide a stratified combustible mixture in the combustion chamber for allowing use of lean air-fuel mixture and for recirculating a large amount of exhaust gas, thus increasing fuel consumption efficiency and decreasing the toxic component in the exhaust gas.
In this type of engine, however, a strong flow of the combustible mixture is introduced into the combustion chamber from the second intake port during the low speed or low load conditions. At such times, the degree of stratification of the combustible mixture in the combustion chamber becomes small, resulting in unstable burning of the lean air-fuel mixture.